Uneven shading in render but not preview (Cycles)
I've had this problem couple of times. I can't find in Blender manual anything about differences between preview and render but as far as I saw it preview isn't accurate like the final render.

In your case clearly geometry causes this shading (as in mine cases). For me only solution was to clean it up.

First of all try to remove doubles but if you want to make it 100% right you will need to retopologize this model.